Articular cartilage, or hyaline type cartilage, is a unique tissue providing a smooth, lubricious, hydrophilic, load bearing covering on the ends of bones in diarthroidal joints, in particular the knee, hip, shoulder, to name a few. This tissue is susceptible to damage or deterioration caused by excessive loading resulting in inflammation, pain, swelling, and joint dysfunction. As a result many methods have been developed to clinically treat patients when cartilage degeneration occurs.
Articular cartilage, or hyaline type cartilage, is a unique tissue providing a smooth, lubricious, hydrophilic, load bearing covering on the ends of bones in diarthroidal joints, in particular the knee, hip, shoulder, to name a few. This tissue is susceptible to damage or deterioration caused by excessive loading resulting in inflammation, pain, swelling, and joint dysfunction. As a result many methods have been developed to clinically treat articular cartilage defects.
For smaller cartilage defects surgical techniques have been used to stimulate an intrinsic repair process. These include drilling, abrasion and microfracture of the subchondral bone which induces bleeding resulting in the formation of a new fibrocartilage covering. Unfortunately the biomechanical properties of this tissue is not equivalent to the original hyaline cartilage, and over time the repair tissue is prone to wear, many times resulting in osteoarthritis.
Alternatively, an osteo-articular transfer system (OATS) procedure may be done, especially as the defect size increases. This technique involves coring a plug of cartilage and subchondral bone from a non weight bearing area of the bone and implanting it to a prepared hole in the defect area. One or multiple plugs can be used to fill the defect area. This procedure is technically difficult as the cored bone/cartilage plugs must be accurately placed to create the new contiguous articulating surface. Leaving the surface of the plugs too high or low can significantly compromise the surgical outcome. Due to the multiple drilling locations and angles needed, this procedure is typically done with an open surgical technique followed by a lengthy rehabilitation schedule.
Autologus chondrocyte implantation is a transfer type system where cartilage cells are harvested in one surgical procedure, expanded in a laboratory, and then injected into the prepared defect site in a second surgery. While clinical outcomes are reported to be similar to the above described techniques this procedure is extremely expensive, requires two surgeries (one of which is a challenging open procedure), and similar long rehabilitation schedule.
Other biological attempts have been made to treat larger cartilage defects with tissue engineered bioabsorbable scaffold systems. Unfortunately they have not shown clinical outcomes advantageous to the above described techniques.
For many larger defects in the knee the only option available is to treat these defects nonoperatively in an endeavor to control symptoms until a unicompartmental knee replacement (UKR) or total knee replacement (TKR) is accomplished. With these devices both articulating bone ends are removed and replaced with metal and an ultrahigh molecular weight polyethylene insert (with or without a metal backing) is placed between the two metallic pieces. In a UKR both bone ends of the medial or lateral half of the knee are replaced whereas with a TKR both halves (and patella) are replaced. These prosthetic devices require an invasive, technically demanding implantation procedure and a long, involved, and painful rehabilitation period. Further, these devices are often larger than the defective tissue that needs to be replaced, so healthy bone and cartilage are sacrificed to accommodate the implants. Albeit that modern UKR and TKR devices are much improved from early hinged knee prostheses, there is still a loss of joint kinematics as this normal tissue is removed. Additionally, the lifetime of TKRs is limited by a variety of implant and patient-related factors resulting in many patients outliving their primary prosthetic device, thus requiring a more difficult revision TKR surgery. To avoid this eventual revision surgery many younger patients will endure the pain and limited use these defects cause in order to put off the TKR procedure as long as possible. It should be noted that the same events occur in the hip and shoulder joints as well.
Implants constructed using measurements obtained from a defect have also been used. The installed implant thus attempts to closely match the shape of the defective area and articulate directly with the opposing native cartilage surface. This device has operative advantages over traditional knee prostheses; however, the opposing articular cartilage is prone to damage due to the large differences in material properties and is further exacerbated by any contour mismatching.
Similarly, metals, usually cobalt-chromium or titanium alloys, have been used for the surface of hip hemiarthroplasties. These prosthetic devices replace only the femoral side of the hip joint and articulate against the facing cartilage of the acetabulum. These metal implants have exhibited adverse effects on the cartilage against which they articulate causing erosion of the facing cartilage in several clinical studies. Thus, merely matching the anatomical shape of the cartilage that is resurfaced is not enough to prevent damage of the facing cartilage by a metallic counterface.
Several researchers have tried using lower modulus polymeric materials, such as high density or ultra high molecular weight polyethylene (UHMWPE), for the surface of hemiarthroplasty implants on the theory that a material with mechanical properties more closely matched to those of cartilage would cause less cartilage damage. These implants also caused erosion of the facing cartilage in vivo likely due to a mismatch in surface chemistry properties, (i.e. UHMWPE is hydrophobic and cartilage is hydrophilic). Thus, lower modulus implants alone are not enough to prevent damage of the facing cartilage.
Accordingly there is a need for an improved cartilage replacement system that would be effective in restoring a smooth, lubricious, and hydrophilic load bearing surface, with a modulus less than traditional metals, that can be easily implanted with minimal normal tissue removal, and requires a less involved rehabilitation schedule ultimately restoring joint kinematics while avoiding damage to the opposing cartilage surface.